Somatostatin or somatotropin-releasing inhibiting factor (SRIF), is a small peptide hormone that inhibits the release of several endogenous hormones including growth hormone (somatotropin), insulin, and thyroid stimulating hormone (Spencer 1986). These hormones, in turn, regulate the release of still other substances. For example, insulin-like growth factor-1 (IGF-1), formerly referred to as somatomedin, and which mediates the anabolic effects of pituitary growth hormone, appears to be released by the liver only when adequate levels of insulin are present (Schalch et al., 1979). Furthermore, the ability of IGF-1 to stimulate growth may be dependent on thyroid hormones (Froesch et al., 1976). Thus, circulating SRIF levels profoundly affect IGF-1 levels.
SRIF may also influence digestion and absorption of nutrients. Molecules with SRIF-like reactivity increase in plasma following a meal (Schusdziarra, 1985). SRIF also inhibits gastric acid secretion, gall bladder emptying and triglyceride and glucose absorption. Thus, SRIF is involved in regulating nutrient uptake from the gastrointestinal tract.
SRIF may influence the reproductive process through several mechanisms. The literature is clear that SRIF does not have a direct stimulatory or inhibitory effect on pituitary secretion of luteinizing hormone (LH) or follicle stimulating hormone (FSH) (see reviews by Brazeau, 1986; Reichlin, 1983). Several studies in farm animal species have confirmed this. Infusion of SRIF in bulls in vivo failed to change levels of LH in blood (Hafs et al., 1977). Injection of ewes with SRIF failed to change serum LH levels and reproductive efficiency (Hoefler and Hallford, 1988). SRIF may influence the reproductive process through other direct or indirect effects. For example, SRIF is present in porcine ovaries (Mori et al., 1984), in the cytotrophoblast of the immature human placenta (Watkins et al., 1980), and in neuroendocrine cells found in the porcine uterus (Vittoria et al., 1989). SRIF also inhibits meiotic maturation of cultured porcine follicular ova (Mori et al., 1985). Without being bound to a particular theory, it is possible that as a consequence of immunization against SRIF, increased pituitary growth hormone secretion and/or increased nutrient absorption which is independent of growth hormone but directly dependent on SRIF, could occur. In either case this could result in increased levels of IGF-1 and altered reproductive performance. It has also been shown that injection of growth hormone in pigs, which results in increases in IGF-1, leads to increased ovulation rates in gilts in estrous (Kirkwood et al., 1988a,b). Similarly, IGF-1 in cultured swine granulosa cells stimulates steroidogenesis (Veldhuis et al., 1985). Further, SRIF may modulate cellular growth by influencing the extent of phosphorylation of the epidermal growth factor (EGF) receptor, histones, and angiotensin (Liebow et al., 1989). Modulation of the responsiveness of the EGF receptor in granulosa cells may regulate cell development and differentiation (Feng et al., 1987).
Immunization against SRIF may also increase reproductive efficiency by affecting cyclic AMP (cAMP) levels. It is known that SRIF inhibits cAMP activity, however, to the knowledge of the inventors, the action of cAMP in reproductive cells and tissues and on ovulation, has not been previously reported. However, cAMP is known to be an important regulator of several physiological mechanisms and the function of cAMP in controlling hormonal activity has been described (Sutherland, 1972). Without being bound to a particular theory, cAMP may regulate reproductive efficiency as follows. cAMP has been implicated in the function of plasminogen activator (Tilly et al., 1990), a neutral serine protease that catalyzes the conversion of plasminogen to plasmin. Specifically, increases in cAMP levels have been shown to induce plasminogen activator activity in the thecal layer of the largest preovulatory follicle in the hen ovary (Tilly et al., 1990). Plasminogen activator, may in turn, affect several physiological processes within the ovary, including cellular differentiation, follicular maturation and ovulation. SRIF, by inhibiting cAMP, may reduce the effect of the cAMP-stimulating factors, luteinizing hormone, and prostaglandin E, thus reducing the effects of these factors on reproductive performance. Since the regulatory function of cAMP has a long evolutionary history extending back to include bacteria (Stryer, L., 1981), it is likely that agents affecting cAMP levels will be useful in a wide range of organisms.
SRIF may also exert an effect on immunological function. Specifically, it has been postulated that SRIF may inhibit lymphocyte proliferation by blocking both RNA and DNA synthesis, Payan et al., 1984. SRIF also exerts inhibitory effects on mononuclear cells, Yousefi et al., 1990, and inhibits the release of IgA by plasma cells isolated from spleen and Peyer's patches, Stanisz et al., 1986. However, the inventors are unaware of any reports indicating that SRIF directly or indirectly inhibits or stimulates cells of myeloid origin, e.g., polymorphonuclear neutrophilic granulocytes (PMN).
It has been reported that lymphocytes, PMN's and monocytes are capable of synthesizing and secreting small amounts of SRIF, Johnson et al., 1985. The exact role of monocyte-secreted SRIF has not yet been described, although Blalock et al., 1985, suggest that this SRIF may function as a signal transmitter between cells of the immune system.
The mechanism by which antibodies to SRIF enhance PMN activity is not known. However, immunization against SRIF may reduce the potential inhibitory effects that SRIF has on the cells of the immune system. The inhibitory effects may be of two types; one may involve the direct interaction between SRIF and PMNs. Alternatively, SRIF may cause the inhibition of lymphokine release by T-lymphocytes. Lymphokines are produced by activated lymphocytes and enhance leukocyte function. Examples of lymphokines are Interferon .alpha., Interferon .gamma. and Interleukin 2. The reduction of lymphokine secretion caused by SRIF would decrease PMN activity. Therefore, the immunization against SRIF may enhance immunological function since many cells of the immune system are regulated by lymphokines, e.g., PMN's, B lymphocytes, macrophages, monocytes and T lymphocytes.
The phagocytic uptake of bacteria by PMN's is the primary step in the clearance of bacteria and as a result PMNs play a pivotal role in the protection of animals against bacterial infection such as mastitis and bovine respiratory disease. Bacterial infection is particularly prevalent in animals under stress, and/or during the periparturient period, and/or following primary viral infection. Thus, PMN activity is a key factor for the prevention and recovery from bacterial infection in numerous situations.
SRIF is a tetradecapeptide which exists in both linear and cyclic forms. The chemical structure of cyclic SRIF-14 is as follows: ##STR1## Molecules with SRIF-like activity also exist with sequences extended from the N- and/or C-terminals as well as molecules which contain additions, deletions, or substitutions within the SRIF-14 sequence. Natural forms of SRIF include the following: SRIF-14;, Pro-SRIF, Pro-Pro-SRIF, SRIF-28, SRIF-25, and SRIF-20.
One well-known analog of SRIF is (SRIF) SMS 201-995. A review of such structures is given by Moreau et al. (1987). Other analogs of SRIF are RC-121 and RC-160 (Liebow et al., 1989).
SRIF has been found in all vertebrates studied to date (Spencer, 1986) and is synthesized throughout the body. Thus, levels of SRIF are not easily reduced and studies regarding modulated SRIF concentrations have been limited. However, immunomodulation has provided a method by which one can examine the effects of anti-SRIF activity. This technique utilizes antibodies to SRIF to modulate SRIF-like activity. Antibodies to SRIF can either be administered directly to the subject (passive immunization) or SRIF can be administered in combination with adjuvants and/or carriers so that antibodies are produced in vivo that can modulate SRIF-like activity (active immunization).
SRIF was originally recognized for its inhibitory effect on growth hormone secretion. Its removal from the blood circulation by passive immunization increased growth hormone concentrations in plasma in rats (Arimura and Schally, 1976), and prevented decreases in growth hormone concentration as a result of stress (Arimura et al., 1976) and starvation (Tannenbaum et al., 1978). However, earlier attempts at active immunization to accomplish immunomodulation of SRIF have produced mixed results. For example, removal of SRIF by active immunization has been reported to result in higher plasma growth hormone levels in lambs (Varner et al., 1980; Spencer et al., 1983a). This, however, resulted in inconclusive effects on growth. Spencer et al. (1983a,b) observed increased growth with improved feed efficiency in lambs immunized against SRIF, whereas Varner et al. (1980) reported a significantly lower growth rate. Furthermore, active immunization against SRIF failed to increase either milk production or growth rates in sheep (Deligeorgis et al., 1988). The often contradictory findings reported in the above references may be explained by the difficulty in obtaining a consistent and measurable immune response against SRIF.
To date, there are no known studies correlating SRIF-like immunomodulation, either active or passive, with reproductive efficiency in animals. U.S. Pat. No. 3,863,008 discloses that increased SRIF levels stimulate luteinizing hormone secretion in vitro, however this effect is not related to an immunological phenomenon. Increased reproductive efficiency in vertebrates would be economically desirable.